A communication system is comprised, at a minimum, of a sending station and a receiving station interconnected by way of a communication channel. A radio communication system is a type of communication system in which the communication channel is formed of a portion of the electromagnetic spectrum. A cellular communication system is exemplary of a multi-user, radio communication system.
Communication capacity in a communication system is, in many instances, limited by the channel capacity of channels available to the communication system. In a radio communication system, for instance, the communication capacity of the system is sometimes limited by the bandwidth allocated to the communication system. Viz., a radio communication system is typically bandwidth-limited. Channels defined in the communication system must be within the bandwidth allocated thereto.
Digital modulation techniques are sometimes used to increase the effective capacity of a communication system. When digital modulation techniques are employed in a radio communication system, for instance, a lessened amount of frequency spectrum is required to effectuate the communication of a communication signal between a sending and a receiving station.
A composite modulation technique is sometimes utilized to form modulated signals. In composite modulation, information is encoded in both the amplitude and the phase of the modulated signal.
In conventional practice, data generated by a data source is provided to a waveform generator. The waveform generator generates digital samples corresponding to the base band I (in-phase) and Q (quadrature-phase) components. The waveforms are generated, for example, in real-time, or are stored in a memory element and selected responsive to input data. The sampling rate and the number of bits per sample of the I and Q-components are selected to represent the signal with sufficient accuracy. The I and Q samples are converted into analog form by digital-to-analog converters (DACs). Once converted into analog form, the samples are filtered by low-pass reconstruction filters. Such filters remove spectrum repetition caused by the sampled nature of the original signal. Once filtered, the I and Q signals are provided to a conventional, quadrature modulator. In conventional practice, a quadrature modulator separates a transmitter carrier source (or a carrier intermediate frequency source) into sine and cosine components. Each component is mixed with separate portions of an information signal input, and the mixed components are then summed. The resultant signal, if at an intermediate frequency (IF), is first up-converted in frequency and then amplified.
Sometimes, the digital-to-analog converter must be of a 10-14 bit resolution to represent the I and Q-components with adequate resolution. Implementation of DACs with such a multi-bit resolution is difficult to implement on the same integrated circuit chip as that upon which digital signal processing components are implemented.
A .DELTA..SIGMA. modulator is sometimes utilized to obviate the need for a DAC of such a high bit resolution. .DELTA..SIGMA. modulators are coupled to receive the I and Q-components formed by the waveform generator. The .DELTA..SIGMA. modulators create streams of digital samples at higher rates than the I and Q-components provided thereto, but each sample created by the .DELTA..SIGMA. modulators are of smaller bit lengths. The reduced number of bits of which the samples formed by the .DELTA..SIGMA. modulators permits the DACs to be of smaller bit resolutions.
Selection of the number of bits of which the samples formed by the .DELTA..SIGMA. modulators can be balanced with the over sampling factor, i.e., the rate at which the .DELTA..SIGMA. modulators generate samples responsive to the I and Q component samples provided thereto. At increased sampling rates, the number of bits of which each sample is formed is reduced. At a great enough over sampling factor, the samples can be merely of single bits thereby making the DACs trivial to implement.
Additional circuit complexity is required of the circuitry which forms the I and Q representation of the base band signal when the signal must be offset slightly in frequency. Frequency offset is required, for instance, to compensate for effects of Doppler shifting. Doppler shifting is sometimes significant in communication systems when sending and receiving stations move rapidly relative to one another. Communications in a satellite communication system, such as a satellite-cellular communication system, or a terrestrial cellular communication system are sometimes affected by Doppler effects. And, compensation must sometimes be made to counteract for the Doppler shifting.
A frequency domain representation of waveforms generated by a waveform generator can be advantageously utilized. That is to say, the waveform generator can be formed which generates output samples corresponding to instantaneous frequency deviations of the modulated signal. Such values of frequency can be integrated, i.e., summed in the digital domain, to obtain phase values. The phase values can be converted to I and Q samples through sine/cosine calculations to transform between polar and Cartesian coordinates. Because the sine/cosine calculations are needed to transform between the polar and Cartesian coordinates, the required circuitry is still of increased complexity.
A manner by which to provide an IQ modulator which permits frequency offsets to be introduced simply and without increasing circuit or processing complexity would be advantageous.
It is in light of this background information related to digital modulation techniques that the significant improvements of the present invention have evolved.